Mechanical differential gear trains, typically referred to as mechanical differentials or simply differentials, are commonly used in many machines, particularly in powered vehicles. The primary purpose of a mechanical differential is to divide torque equally between two driving wheels to permit the wheels to rotate at different velocities when the vehicle turns. Differentials have one input shaft and two output shafts, whereby the input shaft is typically driven by a power source, such as a motor or an engine.
Mechanical differentials exhibit two important properties. First, a mechanical differential causes equal torque to be applied to both output shafts at all times. Second, when a mechanical differential is employed, the average of the two output shaft velocities is always proportional to the input shaft velocity. Therefore, if the input shaft is held fixed, the two output shafts are able rotate at equal velocities but in opposite direction. Similarly, if the first output shaft is held fixed while the input shaft is rotating, the second output shaft will rotate at twice the velocity then it would have if the first output shaft were allowed to rotate at equal velocity to the second output shaft.
Mechanical differentials have a number of drawbacks. For example, mechanical differentials are subject to wear and power loss through the gear sets. In addition, mechanical differentials are not well suited for rapid, incremental motion applications because of the introduction of additional inertial loading due to the gear sets themselves, as well as the inevitable backlash introduced by the gear sets as they wear due to bi-directional loading.
Electric motor systems that employ position control to enable an electric motor to drive a load are well known in the motion control industry. An example prior art motor system 1 is shown in block diagram form in FIG. 1. As seen in FIG. 1, the motor system 1 includes a motor control subsystem 5, which, as described in greater detail below, controls the operation of an electric motor 30, which in turn drives a mechanical load 35. The motor 30 is what is commonly referred to as a servo motor. A servo motor, as that term is used herein, refers to a motor that is controlled based on a closed feedback loop, wherein the feedback is in the form of some motion parameter or attribute of the motor such as rotor position (i.e., angular position), rotor velocity, or rotor acceleration.
As seen in FIG. 1, the motor control subsystem 5 includes a motion profile generator 10, a summing junction 15, a digital filter 20, a power stage 25, and an encoder 40. The motion profile generator 10 generates and outputs a motion profile which is designed to selectively control the angular velocity of the rotor of the motor 30 by controlling the angular position of the rotor over some period of time. In particular, in the embodiment shown in FIG. 1, at some periodic rate (e.g., every 500 microseconds), the motion profile generator 10 injects a desired rotor position into the summing junction 15. The actual rotor position of the motor 30, as provided by the encoder 40 as described below, is subtracted from the desired position to provide a position error. The position error is injected into the digital filter 20 which in turn outputs a DAC (digital to analog converter) value.
In the industry, the digital filter 20 is most commonly a PID (proportional, integral, derivative) controller. It should be appreciated, however, that the digital filter 20 can be any suitable algorithm that converts position error into a DAC value that gets injected into the power stage 25 (also referred to as an amplifier or drive). The output of the power stage 25 is typically electrical current (but can be a voltage) that is provided to the motor 30 that ultimately provides the desired quality of motion at the mechanical load 35. The DAC value is scaled accordingly to match the inputs and outputs of the power stage 25. For example, many commercially available amplifiers use ±10VDC as an acceptable analog input signal. The power stage 25 converts this input signal into and outputs a winding current that is proportional to the input signal.
In lieu of an analog output, the digital filter 75 may output a digital value whereby the power stage 25 can accept this digital value and accomplish the same as the analog version. The winding current is delivered to the motor 30 and is typically proportional to the output torque of the motor 30. This ultimately provides motion to the mechanical load 35. An encoder 40, or other suitable feedback device, is located somewhere on the motor shaft of the motor 30 or on the driven mechanism and provides the actual rotor position of the motor 30 back to the summing junction 15, completing the outer closed loop (the control loop within the power stage 25 is commonly referred to as the inner loop).
FIG. 2 is a block diagram of an implementation wherein the motor 30 of the motor system 1 shown in FIG. 1 is used to drive a mechanical differential 45 operatively coupled to first and second output shafts 50. As discussed elsewhere herein, mechanical differentials such as mechanical differential 45 exhibit two important properties. The first property is that equal torques are applied to both output shafts 50 at all times. Therefore, T1=T2, where T1 and T2 are the output torques of the mechanical differential 45. The second property is that the average of the velocities of the output shafts 50 is always proportional to the input shaft velocity (provided by the motor 30). Therefore, ωi=c*(ω1+ω2)/2, where ωi equals the input velocity to the mechanical differential 45, ω1 and ω2 are the respective output velocities of the output shafts 50, and c is a proportionality constant related to internal gearing/arm ratios within the mechanical differential 45.
As described above, mechanical differentials, such as mechanical differential 45, have a number of drawbacks. Therefore, it would be beneficial to provide the functionality of mechanical differentials in a manner in which the drawbacks of mechanical differentials were eliminated.